1. Field of the Invention
This invention relates to a sealing assembly to be disposed about shafts which are operated at high speed and generate considerable frictional heat. More particularly, this invention is directed to an assembly to be disposed about a rotatable shaft which assembly contains a fluid medium such as an oil wherein there is provided a flexible sealing ring adaptable to be disposed against the shaft and to be in contact with the contained fluid medium which flexible sealing ring has on the surface to be in contact with the fluid medium and, thus, the surface facing the interior of the housing a layer which is impermeable to the fluid medium. The flexible sealing ring has on its opposite surface, the surface in engagement with the shaft, free fiber like ends which are adaptable to be disposed a majority of which fiber like ends are adaptable to be disposed in an axial direction towards the interior of the housing and to be disposed in the direction of rotation of the shaft. This invention is particularly directed to positive-contact seals of the radial or axial lip seal type having a self-adjusting helix.
2. Discussion of the Prior Art
The use of sealing seals has long been known. For instance, it has been known to utilize felt rings or leather shaft seals which are disposed about a shaft and can help in containing a fluid medium. This technology has given way in recent years to the use of elastomeric shaft sheilds. These seals are useful in low rotation speed operations. Particularly, felt and leather seals can be employed where the rotational speed of the shaft is low and only moderate heats of friction are generated. However, the application of these kinds of materials is limited by the rate of revolution of the shaft and by the temperature thusly generated.
For the higher rates of speed, shaft seals of elastomers have proven to be of value. These have a generally better sealing capacity. For this purpose it is necessary that the sealing edge be operated exactly in the range between dry operation and full hydrodynamic lubrication. Strict consideration must be given to the frictional heat which is generated during the rotation of the shaft if one is to preserve the lubricant so that it functions and thus to insure that the sealing material is present during the desired lifetime. With the rise in temperature during such high speed operations the range of application of the elastomeric sealing rings becomes limited. The use of such elastomeric sealing rings is thus quite dependent upon the temperature of the medium.
As all elastomers are not fully suited for dry operation by design, a well lubricated sealing edge which is as narrow as possible is often provided. In order to achieve a perfect sealing, a minimum contact pressure (pre-stressing) of the lip seal is necessary. The frictional heat which is generated causes a high rise in temperature at the narrow contact surface. This means that great precision is required to avoid leakage of the medium. High requirements must be observed if even minimum defects resulting in leakage are to be avoided as a result of the high rise in temperature at the narrow contact surface.
Attempts have been made to produce a sealing lip of a material having a low coefficient of friction. For instance, it has been proposed to utilize polytetrafluoroethylene as a sealing lip material. Such attempts have improved the principal difficulties encountered owing to the development of heat of friction, but they have introduced the disadvantages such as in poor dynamic sealing properties and high wear characteristics which ultimately lead to an early breakdown of the sealing ring. Additionally, the sealing improvement initially obtained as a result of the low coefficient of friction of the polytetrafluoroethylene is observed only in particular cases under particular conditions.
In the case of fast rotating shafts where extreme requirements must be met these sealing lips of polytetrafluoroethylene have not fully met the requirements.
There have already been developed, in respect of high speed rotational shafts improvements in order to wind back leaking oil. For instance, there has been applied to the traditional sealing lips of elastomeric material an additional helix. To wind back leaking oil on the sealing surface, helical ribs and grooves have been molded or ground. The operation of this helix can be compared to the operation of a wind back thread, i.e., the lubricant, e.g., the medium to be sealed, is prevented from leaking through to the atmosphere between the rotating shaft and the sealing lip by means of the action of the helix.
Seals made of elastomers are known and described in, for example, German Offenlegungsschrift 1,916,214. Here a seal is described which achieves the winding back of the sealed medium by supplying the sealing lips with mobile axial extensions which deform according to the direction or torsion due to the frictional movement created by the revolution of the shaft. According to the motion of the shaft, these edges become inclined and wipe off the oil or other medium from the shaft. As these wiping edges are arranged towards the oil side, these and similar embodiments have the disadvantage in that by wiping too well, too little oil reaches the sealing edge. On the other hand, if the wiping effect is too small, the sealing lip again has, as in the traditional radial shaft seal, to take over the sealing effect.
It, therefore, has become desirable to provide a flexible sealing ring which can be employed about a shaft which can be rotated at high speeds as well as at slow speeds and can be employed independent of the extent of frictional heat generated by rotation of the shaft. More particularly, it has become desirable to provide a shaft seal which does not permit excessive oil to diffuse through the point of contact. On the other hand, it has also become desirable to provide such a shaft seal which does not restrain too much oil so that the shaft is disposed against the proper amount of fluid medium at the point it which engages the flexible sealing ring.